In the prior art, a hydrolyzable group end-blocked organopolysiloxane is prepared by reacting an organopolysiloxane having a silanol group at each end of its molecular chain represented by the following general formula (1): EQU (HO)--(R.sub.2 SiO).sub.n --H (1)
with an organosilicon compound of the following general formula (2): EQU (Z).sub.4-a SiR.sup.1.sub.a (2)
or a partial hydrolyzate thereof in the presence of a condensation catalyst. The hydrolyzable group end-blocked organopolysiloxane thus obtained has the following general formula (3). ##STR1##
Herein, R is a substituted or unsubstituted monovalent hydrocarbon group, and n is such a number that the organopolysiloxane of formula (1) may have a viscosity of 10 to 1,000,000 centistokes at 25.degree. C., Z is a hydrolyzable group, R.sup.1 is a substituted or unsubstituted monovalent hydrocarbon group, and "a" is equal to 0 or 1.
The prior art method, however, suffers from several problems including a need of heating to high temperature and a long time of preparation taken until terminal silanol groups are blocked.